A center cam train structure for driving a camshaft which is used in, for example, an inline four-cylinder overhead cam engine or the like for a motorcycle, is disclosed in Japanese Laid-Open Patent Application Publication No. Sho. 60-243307. FIG. 1A is a plan view of a cylinder head 5A having a conventional center cam train structure. The center cam train structure is configured in such a manner that a chain line unit 4 disposed at a center of four cylinders 1 (indicated by broken-line circles in FIG. 1A) aligned in the width direction of cylinder head 5A drives two camshafts 2 that are provided with cams 3 and are respectively configured to drive an intake valve and an exhaust valve.
It is necessary to fasten cylinder head 5A to a cylinder block (not shown) located thereunder firmly by bolts 6 at opposite sides of each cylinder 1, in order to withstand the stress generated during combustion in each cylinder 1.
In the above-described center cam train structure, however, the chain line unit 4 provided with a sprocket 4a is disposed at the center, and the bolts 6 cannot be disposed at a region of the cylinder head 5A that is protruded to conform in shape to the sprocket 4a. For this reason, the bolts 6 are forced to be disposed at opposite sides with respect to the sprocket 4a in the width direction of the cylinder head 5A, and the width of the entire cylinder head 5A may increase by a distance B between the bolts 6 disposed at opposite sides of the sprocket 4a in the width direction of the cylinder head 5A. The width of the entire cylinder head 5A is equal to a length indicated by “4A+B” in FIG. 1A.
To solve the above described problem, a side cam train structure has been commonly used. FIG. 1B is a plan view of a cylinder head having a conventional side cam train structure. In the side cam train structure, the chain line unit 4 is positioned at an end portion of cylinder head 5B. Advantageously, the side cam train structure allows the width of cylinder head 5B to be substantially smaller than that of cylinder head 5A by a length “B′,” that is typically 20 to 30 mm, approximately.
The reduction of the width of the cylinder head is favorable for a motorcycle having a frame structure such as a double cradle frame structure, in which a main frame has a pair of tank rail frame members.
The pair of tank rail frame members has a large rectangular cross-sectional area. Spaced apart from each other, the pair of tank rail frame members extends rearward and downward from a head pipe. A fuel tank is disposed to extend along and above the tank rail frame members. The cylinder head is typically disposed between the pair of tank rail frame members. If the distance between the pair of tank rail frame members increases, then stiffness, in particular longitudinal stiffness and torsional stiffness of the entire frame of the motorcycle increases, but toughness of the entire frame decreases, making it difficult for a rider to turn and steer the motorcycle.